1. Field of the Invention
This invention relates to devices for monitoring electrical current in electrical equipment, and particularly to a device that will sense dc fault current and output an appropriate signal.
2. Prior Art DC Fault Current Sensors
In solid state power conversion equipment, fuses are usually inserted in the dc feed buses of the inverter bridges. This is to provide protection against short circuit current faults and to prevent destruction of the equipment. However, fault current in the dc feed buses can still destroy the solid state switches before the fuses can blow if it is fast fault current. It has therefore become common practice to incorporate de fault current sensors in the dc feed buses of inverter bridges. These dc current sensors function to detect a current fault, and output a signal which can be used by the controls to turn off the solid state switches, thus protecting the switches.
Present day dc fault current sensors, which are required to be installed in the dc feed of a solid state inverter bridge, utilize a special magnetic core that is shaped to fit around a given dc bus, and must be sized for the expected operating current, including faults. A Hall device is placed in the magnetic core aperture and is connected to a Hall bias supply and Hall amplifier. A fault current in the dc bus is sensed by the Hall device, which outputs a signal to the amplifier. The amplifier accepts the device signal and outputs an analog signal, indicating a fault current.
The use of a Hall device type dc current sensor carries with it certain disadvantages and creates design problems For the electrical equipment engineer. Since the sensor is a proprietary design, a vendor restriction is placed on the device configuration, dictating the space required for its installation. This often causes considerable difficulty where space is tight.
The Hall device drifts with temperature changes, requiring the use of complex temperature compensation circuitry for the amplifier. The amplifier must also tolerate high common mode coupling between the magnetic core and the Hall device, requiring a complex instrumentation.
The Hall bias supply must furnish high current for proper Hall device function, which operates at low efficiency. This adds a burden to the electrical equipment power supply and additional heat to be dissipated in usually close quarters.
Magnetic effects after load steps may cause false output signals, requiring some form of filtering out these false signals to be added.
Long term drift effects due to the Hall device with time, must becalibrated else the device will become inaccurate.
A final disadvantage is the low reliability of the device, relative to other types of sensors (e.g., voltage, temperature etc.), due to a high component count. Thus, there exists a need for a simple dc current sensor which does not exhibit the problem areas described above; that does not restrict a power inverter designer to particular configurations; that is reliable and is relatively inexpensive.